feat: recommend server-generated mixes from Musify and YouTube Music

Mixes are their own concept, not playlists: they live in their own
Mixes/MixItems tables, cannot be edited and never show up among the
user's playlists. Each item is either a Musify track or a YouTube Music
reference, so YouTube songs stream directly instead of being downloaded.

A daily Hangfire job seeds the mixes from the listening history: one
discovery mix, one daily mix and one per top artist.

The per-user YouTube filler query was already in the working tree and
rides along here because it shares files with this change.

backend/Musify.Domain/Entities/Mix.cs

@@ -0,0 +1,42 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+using Musify.Domain.Abstractions;
+
+namespace Musify.Domain.Entities
+{
+#pragma warning disable CS8618
+    [Table("Mixes")]
+    public class Mix : IAuditable
+    {
+        [Key]
+        public Guid Id { get; set; } = Guid.NewGuid();
+
+        [Required]
+        public required long UserId { get; set; }
+
+        [Required]
+        [MaxLength(100)]
+        public required string Title { get; set; }
+
+        [MaxLength(256)]
+        public string? Subtitle { get; set; }
+
+        [Required]
+        public required DateOnly GeneratedOn { get; set; }
+
+        [Required]
+        public int Position { get; set; } = 0;
+
+        [Required]
+        public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+        [Required]
+        public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
+
+        [ForeignKey(nameof(UserId))]
+        public User User { get; set; }
+
+        public ICollection<MixItem> Items { get; set; } = new List<MixItem>();
+    }
+}

backend/Musify.Domain/Entities/MixItem.cs

@@ -0,0 +1,50 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+using Musify.Domain.ValueObjects;
+
+namespace Musify.Domain.Entities
+{
+#pragma warning disable CS8618
+    [Table("MixItems")]
+    public class MixItem
+    {
+        [Key]
+        public Guid Id { get; set; } = Guid.NewGuid();
+
+        [Required]
+        public required Guid MixId { get; set; }
+
+        [Required]
+        public int Position { get; set; } = 0;
+
+        [Required]
+        public required MixItemSource Source { get; set; }
+
+        public Guid? TrackId { get; set; }
+
+        [MaxLength(64)]
+        public string? VideoId { get; set; }
+
+        [MaxLength(300)]
+        public string? Title { get; set; }
+
+        [MaxLength(300)]
+        public string? Artist { get; set; }
+
+        [MaxLength(512)]
+        public string? ThumbnailUrl { get; set; }
+
+        [Required]
+        public int DurationSeconds { get; set; } = 0;
+
+        [Required]
+        public bool IsExplicit { get; set; } = false;
+
+        [ForeignKey(nameof(MixId))]
+        public Mix Mix { get; set; }
+
+        [ForeignKey(nameof(TrackId))]
+        public Track? Track { get; set; }
+    }
+}